1. Field of the Invention
The present invention relates to a compatibility, dispersion and/or grinding agent for aqueous pigmentary suspensions formulated from a mixture of mineral pigments, at least one of which is hydrated calcium sulfate. The invention also relates to aqueous pigmentary suspensions containing the dispersion and/or grinding agent formulated from a mixture of pigments, at least one of which is hydrated calcium sulfate, a method for preparing such suspensions, a method of papermaking, using the above-identified aqueous pigmentary suspensions, and the paper made by such a process.
2. Background of the Invention
When aqueous pigmentary suspensions are prepared from a mixture of mineral pigments, there frequently occurs an extremely rapid and considerable increase in their viscosity, which can reach the solidification point of said suspensions.
This phenomenon, which is linked to the incompatibility of the pigments with one another, occurs for example in suspensions containing a mixture of CaCO.sub.3 and titanium oxide, CaCO.sub.3 and kaolin, and CaCO.sub.3 and titanium oxide and kaolin.
Known to the skilled artisan by the term "shock effect", this viscosifying effect also occurs, but to a greater extent, in the case of aqueous pigmentary suspensions containing both hydrated calcium sulfate and calcium carbonate, or even hydrated calcium sulfate, kaolin and calcium carbonate.
Hydrated calcium sulfate is a mineral pigment which has not yet been greatly exploited as such, but possesses interesting qualities in the field of mass charge or coating of the paper with a view to improving the opacity, shine, reaction to water, printing and whiteness thereof (Patents FR 2,370,124; FR 2,343,082; FR 2,440,437; FI 67.568).
In addition, for the satisfactory use of aqueous pigmentary suspensions formulated from a mixture of mineral pigments, at least one of which is hydrated calcium sulfate, it is desirable to be able to use an agent which is capable of both promoting the dispersion of the pigmentary particles and of eliminating the shock effect by rendering the pigments compatible with one another.
For many years the specialized literature has discussed the research carried out to eliminate the shock effect occurring between the hydrated calcium sulfate and other pigments when they are used together in aqueous pigmentary suspensions.
A first reference (EP 0 216 516) describes a compatibility agent which enables the preparation of aqueous suspensions containing as pigments a mixture of gypsum (in which 70% of the particles have a size of less than 2 microns), kaolin (in which 80% of the particles have a size of less than 2 microns) and calcium carbonate (in which 86% of the particles have a size of less than 2 microns). The recommended compatibility agent is formed from an alkylphosphate ester or a polyacrylate, in which at least 15% of the carboxylic sites are esterified.
In this manner, aqueous suspensions containing a mixture of hydrated calcium sulfate and calcium carbonate or kaolin were prepared in the presence of the above-identified compatibility agent, whether it be an alkylphosphate ester or a partially esterified polyacrylate. However, in order to achieve reasonable viscosities for industrial use, the concentration of these suspensions was unable to go beyond 56% by weight of dry material.
In addition and in the case of the use of an alkylphosphate ester as the compatibility agent, there appears a new restriction of having to operate in a medium with a pH of at most equal to 7 in order for the rheology of the pigmentary suspension to remain within acceptable values for the applications planned. Moreover, when the pH exceeds 7, it is no longer possible to control the viscosity of the mixture of pigments in suspension, with this viscosity increasing very rapidly in a detrimental manner.
Therefore, as the aqueous pigmentary suspensions for the coating of the paper must normally have a concentration of dry material of at least 65% and a pH of between 8 and 11, the compatibility agent recommended in said patent is unusable in the field of paper coating.
In EP 0 227 623, a compatibility agent is proposed which is selected from the acid alkylamine salts in order to regulate the viscosity of aqueous pigmentary suspensions intended for paper coating and containing a mixture of calcium sulfate, kaolin and/or calcium carbonate. However, it has not been possible to bring the content of dry material of these suspensions to a value greater than 56% by weight without causing an increase in viscosity which is prohibitive for paper coating applications.
FR 2,370,124 discloses the use of a compatibility agent to stabilize aqueous pigmentary suspensions intended for paper coating and containing a mixture of aluminum trihydroxide and dihydrated calcium sulfate with a grain size of less than 20 micrometers, with said pigment compatibility agent being a phosphate-ester of depolymerized starch, possibly in combination with a conventional dispersion agent.
However, such pigmentary compositions can be used in the paper coating field only if the user is satisfied with a low content of dry material (on the order of 50% by weight) in order to control the rheology, which fact is certainly a disadvantage.
GB 2,173,781 discloses, for the manufacture of a hydrated calcium sulfate slurry, to use as a dispersing agent a mixture of a surfactant agent with a high concentration, selected from the group of organic sulfates or sulfonates, and a complexing agent selected from the inorganic phosphates and the amines. However, as the present applicants have found, such a mixture of surfactant and complexing agents does not enable the compatibility sought between the hydrated calcium sulfate and the other pigments normally used for paper coating to be achieved.